At the Atlantic, sociology prof Philip Cohen provides this graph:
Looking at it from the perspective of 1990, it was easy to assume a strong causal relationship between the rise in single motherhood and the murder epidemic. By my reading of the research, it is true that children of single mothers are more likely to commit crimes. But other factors are more important.
Contrary to Mitt Romney, it seems unlikely that increasing marriage is, in the current situation, the best way to reduce violent crime.